The pineal hormone melatonin and its major urinary metabolite 6-hydroxymelatonin are being quantified by gas chromatography-negative chemical ionization mass spectrometry. Assay of the urinary metabolite as an index of pineal gland activity has been validated by studies in monkeys on the effects of light and pinealectomy. Studies of the daily excretion rates of children have revealed that the pineal produces quantities of melatonin comparable to those of adults from the earliest ages measured (3 years). In boys, there is not significant change in daily 6-hydroxymelatonin excretion with respect to pubertal stage, but in girls, a significant elevation coincident with the onset of breast development was observed. Longitudinal studies are in progress to confirm these observations and relate them to changes in gonadotrophins. The precursor of melatonin, serotonin, has been quantified in monkey csf and marked diurnal variation observed.